A surround is a flexible annular ring connecting between the basket and the cone and provides a movable support for the speaker driver cone. Surrounds are commonly made from compressed foam or molded elastomer. As the driver cone is displaced by the force of the voice coil along the axis of travel, the surround is stretched from its resting position. The force required to move the cone and surround changes as a function of displacement distance in a non-linear fashion. This causes audio distortion. This non-linear effect is particularly evident in high excursion drivers because the non-linearity of the force/displacement relationship increases at high displacements.
The surround and gasket are normally two separate parts.